Both promote sleep and relaxation; the combined sedative effect may be strong.
GABA is an inhibitory neurotransmitter. Melatonin regulates sleep-wake cycles. Together they may cause excessive sedation.
Start with lower doses if combining. Avoid with alcohol or prescription sleep aids.
Known drug category interactions for these supplements. Especially relevant if you take prescription medications.
Melatonin may affect blood pressure and interact with some antihypertensive drugs.
Monitor blood pressure if combining. Some people experience higher blood pressure with melatonin — effects vary.